1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive device comprising a ceramic substrate and at least a piezoelectric/electrostrictive element stacked on the ceramic substrate by means of a film formation method, and more particularly to a piezoelectric/electrostrictive device comprising a plurality of piezoelectric/electrostrictive layers and a plurality of electrode layers including a piezoelectric/electrostrictive material stacked alternately in a comb like structure on a ceramic substrate and to a method for producing the same.
2. Description of the Related Art
In a piezoelectric/electrostrictive device such as an actuator element and a sensor element including a piezoelectric/electrostrictive layer, firstly, a wiring pattern, which is composed of one electrode layer, is formed on a ceramic substrate by, printing for example. Secondly, the piezoelectric/electrostrictive layer is further formed on the wiring pattern by printing to secure the wiring pattern and the piezoelectric/electrostrictive layer to the ceramic substrate by sintering. After that, a wiring pattern, which is composed of the other electrode layer, is formed.
The piezoelectric/electrostrictive device is used as an actuator element in which an electric field is applied to the piezoelectric/electrostrictive layer by supplying an electric signal to the wiring pattern, and the piezoelectric/electrostrictive layer is consequently displaced. Additionally, the piezoelectric/electrostrictive device can be used as a sensor element in which an electric signal, which is generated depending on a pressure applied to the piezoelectric/electrostrictive layer, is extracted from the wiring pattern.
The piezoelectric/electrostrictive device as described above involves the following fear. That is, for example, the wiring pattern in the lower layer undergoes any thermal shrinkage, and a part of the wiring pattern is evaporated when the piezoelectric/electrostrictive layer is sintered. As a result, a lot of unnecessary pores appear in the wiring pattern and pores having large opening areas appear to decrease the portion (conductive portion) which actually functions as the electrode layer.
In such a situation, the area of the electrode layer is substantially decreased. Therefore, the capacitance is decreased and the driving force is lowered. Further, when unnecessary pores are generated irregularly due to the dispersion in the production, then the electrode areas of individual devices are dispersed, and the capacitance is also dispersed. This results in decreasing yield of the devices. Further, it is necessary that the control voltage is adjusted for every individual device when the device is used. A problem occurs in that such a device is difficult to be used (difficult to be controlled).
Further, the adhesive force is weak between the wiring pattern and the piezoelectric/electrostrictive layer. Therefore, any exfoliation occurs in some cases during the machining (for example, cutting and polishing) and/or during the washing (washing with ultrasonic wave) of the piezoelectric/electrostrictive device. In the case of the device as described above, the place, at which the exfoliation occurs, is the interface between the wiring pattern and the piezoelectric/electrostrictive layer.
Further, a portion, in which the electrode layer is not formed on the ceramic substrate, is provided in relation to the planar shape of the wiring pattern. At this portion, the ceramic substrate and the piezoelectric/electrostrictive layer are opposed to one another. However, any gap appears at the portion after the sintering, for the following reason. That is, it is difficult to join constitutive materials of the ceramic substrate and constitutive materials of the piezoelectric/electrostrictive layer.
If such a gap is formed, a part of the piezoelectric/electrostrictive layer consequently floats over the electrode layer. Such a portion exists as a region which is not restricted by the ceramic substrate. As a result, any movement tends to occur due to any external force in this structure. Therefore, the exfoliation is apt to take place.
In view of the above, in order that the piezoelectric/electrostrictive layer and the wiring pattern are not exfoliated from the ceramic substrate, for example, it is necessary that the step of cutting the piezoelectric/electrostrictive device is performed under a condition in which the load on the piezoelectric/electrostrictive layer or the like is decreased. That is, the cutting step is restricted by the condition in which the machining load is small. As a result, the machining time is prolonged, and the throughput is lowered.
On the other hand, it is necessary that the washing step is also performed under a condition in which the load on the piezoelectric/electrostrictive layer or the like is decreased. Therefore, in order to eliminate the dirt, it is necessary to use a longer period of washing time. As a result, the number of steps is increased.
When the exfoliation of the piezoelectric/electrostrictive layer or the like occurs, the following harmful influences also appear.
(1) The function as the piezoelectric/electrostrictive device is deteriorated.
(2) When the wiring pattern is exfoliated from the piezoelectric/electrostrictive layer, the capacitance is decreased. As a result, the amount of generation of strain is decreased in the piezoelectric/electrostrictive layer, and the displacement is decreased.
(3) When the wiring pattern is exfoliated from the ceramic substrate, then the strain, which is generated in the piezoelectric/electrostrictive layer, is hardly transmitted to the ceramic substrate, and the displacement is decreased.
(4) The strength of the entire piezoelectric/electrostrictive device is decreased, and the resonance frequency is lowered when the piezoelectric/electrostrictive device is used as an actuator element.